Metal halide arc discharge lamps are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide arc discharge lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains a fill material including mercury, metal halide additives and a rare gas to facilitate starting. In some cases, particularly in high wattage lamps, the outer envelope is filled with nitrogen or another inert gas at less than atmospheric pressure. In other cases, particularly in low wattage lamps, the outer envelope is evacuated.
It has been found desirable to provide metal halide arc discharge lamps with a shroud which comprises a generally cylindrical, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends. In other cases, the shroud is open on one end and has a domed configuration on the other end. Shrouds for metal halide arc discharge lamps are disclosed in U.S. Pat. No. 4,499,396 issued Feb. 12, 1985 to Fohl et al and U.S. Pat. No. 4,580,989 issued Apr. 8, 1986 to Fohl et al. See also U.S. Pat. No. 4,281,274 issued Jul. 28, 1981 to Bechard et al.
The shroud has several beneficial effects on lamp operation. In lamps with a gas-filled outer envelope, the shroud reduces convective heat losses from the arc tube and thereby improves the luminous output and the color temperature of the lamp. In lamps with an evacuated outer envelope, the shroud helps to equalize the temperature of the arc tube. In addition, the shroud effectively reduces sodium losses and improves the maintenance of phosphor efficiency in metal halide lamps having a phosphor coating on the inside surface of the outer envelope. Finally, the shroud improves the safety of the lamp by acting as a containment device in the event that the arc tube shatters.
In the aforementioned U.S. Pat. No. 4,580,989, two techniques for mounting the shroud are disclosed. According to one technique, the shroud is retained by two metal straps which encircle the shroud and are welded to a frame. According to the second mounting technique, the open end of the shroud is provided with a pair of diametrically opposite slots. The slots engage a metal strap used for mounting of the arc tube.
While both prior art configurations provide generally satisfactory performance, they have been found to have certain disadvantages, particularly in a high volume production environment. In the construction which utilizes a pair of straps which encircle the shroud, the shroud tends to shift lengthwise and/or rotate in the straps when the lamp is jarred during shipping or handling. In the construction which utilizes a pair of slots for engaging a lamp mounting strap, the shroud is subject to cracking or breakage in the areas of the slots where the shroud contacts the strap. Furthermore, it is necessary to heat treat the shroud in the region of the slots to reduce the possibility of cracking or breakage. The heat treatment step adds to the production cost. Accordingly, it is desirable to provide a metal halide arc discharge lamp wherein the shroud is securely locked in position, while cracking or breakage of the shroud is avoided.
It is a general object of the present invention to provide improved electric lamps and, more particularly, improved metal halide arc discharge lamps.
It is a further object of the present invention to provide an electric lamp assembly wherein a light-transmissive shroud is securely locked in position. It is another object of the present invention to provide a metal halide arc discharge lamp wherein the need for slots in a light-transmissive shroud is eliminated.
It is yet another object of the present invention to provide a metal halide arc discharge lamp having a light-transmissive shroud which is protected against movement, cracking and breakage during shipping and handling.
It is still another object of the present invention to provide a metal halide arc discharge lamp which is low in cost and is easily manufactured.